1. Field of the Invention
This invention relates to a lens system consisting of three lens elements, and more particularly to a lens system which consists of three lens elements and is suitable for use as an image pick up lens system for a video camera or a still-video camera for a visual phone, a door phone, monitoring or the like.
2. Description of the Prior Art
Recently a solid state image pickup device has been in wide use in various video cameras or still-video cameras. The solid state image pickup device has been made smaller year by year and accordingly there has been a demand for miniaturization of the lens system and for reduction in cost of the lens system.
In video cameras and still-video cameras, a low-pass filter for avoiding moire patterns, an infrared cut filter for correcting the spectral sensitivity of the solid state image pickup device and/or a cover glass for protecting the image surface of the image pickup device are generally disposed between the lens system and the solid state image pickup device. In such a case, the lens system should have a long back focal length in order to accommodate such optical elements.
Recently there has been put into practice a solid state image pickup device in which a micro convex lens is disposed in front of each light receiving element of the solid state image pickup device in order to accumulate a light bundle travelling toward the dead zone of the element to the sensitive zone of the element, thereby increasing the sensitivity of the solid state image pickup device. When the light bundle entering such a solid state image pickup device largely inclines with respect to the optical axis of the micro lens, a so-called eclipse occurs at the aperture of the micro lens and a part of the incident light cannot reach the light receiving element. As a result, the brightness of the image becomes poor in the peripheral portion of the image and the peripheral portion of the image becomes dark. In order to avoid such a phenomenon, the incident angle of the incident light bundle to the solid state image pickup device should be as small as possible and it is necessary to position the exit pupil of the lens system as far from the image plane as possible.
In our Japanese Patent Application No. 5(1993)-314649, there is disclosed a two-lens image pickup lens system for a video camera or a still-video camera. The lens system comprises a meniscus lens element having a negative refractive power and a lens element having a positive refractive power and satisfies a predetermined condition. This lens system is long in the back focal length, clear and relatively wide in the angle of view and at the same time is small in size. However in the lens system, the incident angle of the incident light bundle to the solid state image pickup device is large and an eclipse occurs at the aperture of the micro lens when this lens system is used with a solid state image pickup device with micro lenses.
Further when the exit pupil of an image pickup lens system is positioned far from the image plane, correction of chromatic aberration becomes difficult.